Runoff response to land management in small catchments near Lake Tana, Ethiopia

The highlands of Ethiopia are severely affected by soil erosion by water. Consequently land management practices such as stone bunds have been widely implemented in recent years. Here, we investigate the effect of stone bunds and vegetation on steep slopes on quick flow (direct flow due to rain storm) in six small catchments (20 – 80 ha), which are dominantly agriculture and have seasonal streams. Hydrological monitoring stations were installed at the outlet of each catchment, and discharge and rainfall were recorded every five minutes during the rainy seasons of 2018 and 2019. Event-based quick flows (Qd) were analyzed against rainfall (P) and catchment characteristics. Quick flow was also predicted with the Natural Resources Conservation Service “Curve Number” (NRCS-CN) method, which is an empirical model used to predict runoff based on hydrologic soil group, land use and antecedent moisture condition. Calibration of the NRCS-CN runoff model based on observed Qd and P resulted in an abstraction coefficient (λ) (which is a ratio of the initial abstraction to maximum potential retention) value of 0.014, much less than the commonly used 0.05. CNs in July and August are higher than June and September due to high antecedent moisture condition and showed declining trend with increasing stone bund densities. In catchments with high density of stone bunds and vegetation on steep slopes the quick flow was reduced by 43% compared to catchments without conservation structures; implying that stone bunds and vegetation rehabilitation implemented so far have been effective in reducing quick flow and enhancing soil infiltration and ground water recharge. Based on our results, a value of λ = 0.01 is recommended for runoff prediction using the NRCS-CN method in the sub-humid Ethiopian highlands, rather than the commonly accepted 0.05. Besides, ongoing efforts of soil and water conservation in the highlands of Ethiopia should be promoted in catchments where quick flow is high due to land degradation